Conventionally, as a pressure sensitive sensor to measure a pressure of an external force (also referred to as pressed force) applied to a certain surface, there is a sensor as disclosed in a patent literature 1 (Japanese Unexamined Patent Publication No. 2002-48658). According to the sensor in the patent literature 1, plastic films each formed by laminating an electrode and a pressure sensitive ink layer in this order sandwich an insulating layer having bonding layers on its front and back surfaces in such a manner that their pressure sensitive ink members are opposed to each other. In addition, according to the sensor in the patent literature 1, since irregularities are provided on a surface of the pressure sensitive ink layer, a certain distance gap is formed between the upper and lower pressure sensitive ink layers, and the upper and lower pressure sensitive ink layers are prevented from being closely adhered to each other when a pressure is not applied.
According to the sensor having the above structure in the patent literature 1, when the pressed force is applied to the upper film, the electrode of the upper film corresponding to a part to which the pressure is applied due to the bending of the upper film is brought into contact with the electrode of the lower film through the pressure sensitive ink layer. Thus, both electrodes are conducted. According to the sensor in the patent literature 1, a pressure applied to the upper film can be measured by detecting the conducted state of both electrodes, and a fluctuation in resistance value based on the pressure applied to the pressure sensitive ink layer. When the sensor of the patent literature 1 is mounted, for example, inside a car sheet, it can be determined whether or not a passenger sits on the sheet, and a physical size of the passenger can be determined from a pressure distribution.
Recently, as for an electronic device having a touch panel, especially a portable electronic device such as a portable telephone or a game machine, a pressure sensitive sensor is required to be mounted on the touch panel of a display, for example, instead of an Enter button. In general, the portable electronic device is configured such that a display part of a display device positioned in a casing can be viewed through the touch panel. The sensor in the patent literature 1 is configured such that the electrodes and the pressure sensitive ink layer are arranged in large parts of the surfaces of the upper and lower films, so that transmittance (visibility) is low. Therefore, the sensor in the patent literature 1 cannot be applied to the electronic device having the touch panel as it is.
Thus, it is proposed by the applicants of the present invention that a pressure sensitive sensor which will be described below is attached to a lower surface of the touch panel to prevent visibility of the display part of the display device from being lowered even when the pressure sensitive sensor is mounted in the electronic device having the touch panel {Japanese Patent Application No. 2008-330284 (Japanese Examined Patent Publication No. 4642158}). That is, a pressure sensitive sensor 20 includes a lower film 22, an upper film 21 arranged so as to be opposed to the lower film 22, a pair of electrodes 21a and 22a arranged on opposed surfaces of the films, pressure sensitive ink members 23a and 23b arranged so as to be spaced from the pair of electrodes, and having electric characteristics changed by a pressed force applied, and a gap holding member 24 to bond the upper and lower films 21 and 22 and to hold a gap between the pressure sensitive ink members 23a and 23b and the pair of electrodes 21a and 22a, in which the pair of electrodes 21a and 22a are arranged in the shape of a frame along edge parts of the upper and lower films 21 and 22, and the pressure sensitive ink members 23a and 23b are dotted along the edge parts of the upper and lower films 21 and 22, and when the pressure sensitive sensor 20 is deformed due to an external force applied, at least one of them is brought into contact with the pair of electrodes 21a and 22a to conduct them (refer to FIG. 9).
In addition, according to the pressure sensitive sensor 20, a bump (projection) 26 is laminated and arranged on the back surface side of the lower film 22 just under the position of the pressure sensitive ink member, as a stress concentration member (refer to FIGS. 8 and 9). Thus, the pressure sensitive sensor 20 is attached on the lower surface of the touch panel and mounted in the casing, so that when a pressed force is applied to the pressure sensitive sensor 20 in a thickness direction, a part corresponding to the pressure sensitive ink member 23b of the lower film 22 is supported from below, and the pressed force applied can be surely transmitted as the force used for the deformation of the lower film 22 without being dispersed. Thus, pressure measurement accuracy can be improved.
Patent Literature 1:
Japanese Unexamined Patent Publication No. 2002-48658